The chemistry of polyazamacrocycles with coordinating side arms, which increase the ligating ability of the macrocycles, has developed quickly over the last decade (P. V. Dernhardt and G. A. Lawrance, Coord. Chem. Rev., 1990, 104,297) . Derivatives of 1,4,7,10-tetraazacyclododecane (TAZA) which contain additional donor groups, have been widely investigated due to the applications found for some of their metal complexes. Relevant examples are given by the use of the Gd complex of 1,4,7,10-tetraazacyclododecanetetraacetic acid (DOTA) as a contrast agent for in vivo magnetic resonance imaging (M. Magerstaedt, O. A. Gansow, M. W. Brechbiel, D. Colcher, L. Baltzer, R. H. Knop, M. E. Girton Naegele and M., Magn. Reson. Med., 1986, 3, 808) and of .sup.90 Y complexed DOTA derivatives attached to monoclonal antibodies in radioimmunotherapy (D. Parker, Chem. Soc. Rev., 1990, 19, 271; S. V. Deshpande, S. J. De Nardo, D. L. Kukis, M. K. Moi, M. J. McCall, G. L. De Nardo and C. F. Meares, J. Nucl. Med., 1990, 31, 473). Well known are derivatives of TAZA bearing four identical residues on the nitrogen atoms. On the contrary, TAZA derivatives containing different. coordinating side arms on the nitrogen atoms have received little attention, likely due to the difficulties involved in their synthesis. In this context, 1,7-disubstituted-1,4,7,10-tetraazacyclododecanes can be very useful in order to obtain chelating agents with prefixed symmetry. These compounds can, in principle, be synthesized (T. A. Kaden, Top. Curr. Chem., 1984, 121, 154) by classical condensation according to Richman and Atkins (J. E. Richman and T. J. Atkins, J. Am. Chem. Soc., 1974, 96, 2268; T. J. Atkins, J. E. Richman and W. F. Oettle, Org. Synth., 58, 86). However, the nature of the residues, which can be introduced into positions 1 and 7 by this synthetic approach, is severely limited by the harsh conditions required, in particularly during the deprotecting steps.